1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background Art
There is nowadays a trend to lower, in steps, power supply voltages of semiconductor devices in order to reduce electric power consumption. In actuality, in these past 20 years, the power supply voltage has changed from 5 V to 3.3 V (or 2.5 V), and then from 3.3 V (or 2.5 V) to 1.8 V, except for semiconductor devices for special applications. Currently, the power supply voltage is in a transition period of shifting from 3.3 V (or 2.5 V) to 1.8 V. In a semiconductor device used in a portable device such as a cellular phone, the need to reduce the amount of electric power consumption is particularly strong, and it is predicted that the power supply voltage will decrease from 1.8 V to 1.5 V from hereon.
There are various technological problems in decreasing the power supply voltage. Accordingly, it is difficult to uniformly lower the power supply voltages for all semiconductor devices existing within a system. In the transition period of reducing the power supply voltage, a state arises in which the power supply voltages of some semiconductor devices are relatively low, whereas the power supply voltages of the remaining semiconductor devices are relatively high. In such a transition period of changing the power supply voltage, semiconductor devices corresponding with a dual range power supply voltage, or semiconductor devices corresponding with a wide range power supply voltage are needed.
In order to realize stable operation, the semiconductor device that corresponds with the dual range or wide range power supply voltage changes operation of a circuit, such as a booster or an input/output buffer circuit, according to a magnitude of an external power supply voltage. In many of such semiconductor devices, latching of the power supply voltage during operation of the semiconductor device is often carried out in order for the circuit operation to not become unstable even if the power supply voltage varies somewhat due to noise.
However, in the conventional semiconductor device, when fluctuations in the power supply voltage are relatively great such as a momentary service interruption, operations of the circuits such as the booster or the input/output buffer circuit are changed in the same way as at the time when the power supply voltage is changed. In particular, when the power supply voltage fluctuates momentarily, because a certain amount of time is needed in order to change the operations of these circuits, there are cases in which the circuit, such as the booster or the input/output buffer circuit, carries out operation corresponding to a low voltage range, regardless of the fact that the power supply voltage is actually at a high voltage range. In this case, within the circuit, such as the booster or the input/output buffer circuit, that uses the voltage obtained by boosting the power supply voltage, the power supply voltage of the high voltage range may be boosted by a boosting ability corresponding to the low voltage range. As a result, a voltage that is higher than supposed is generated within the circuit, such as the booster or the input/output buffer circuit, and the circuit may break.
In order to cope with such a problem, there is a technique of, after once latching the power supply voltage during operation of the semiconductor device, maintaining this latched state until the power source is turned off. However, in this technique, the power source must be turned off once in order to change the voltage range of the power supply voltage (see Japanese Patent Laid-Open No. 2003-429150).
There is therefore desired a semiconductor device that can smoothly change an operating state when the range of the power supply voltage is changed, while maintaining a stable operating state even if the power supply voltage varies due to noise or a momentary service interruption or the like during operation.